1. Field of the Invention
The present invention concerns link adaptation and more particularly, techniques for link adaptation based on mutual information per coded bit metrics.
2. Description of the Related Art
Next generation cellular systems support multiple transmission modes, which can be used to improve the performance of such systems by adapting to current channel conditions. This process is referred to as link adaptation. Typically, these transmission modes include different modulation and coding schemes (MCS) and different multiple antenna arrangements—like beamforming, space-time coding and spatial multiplexing—as the transmission becomes simultaneously multi-dimensional in space, time and frequency domain.
To achieve the system-level gain promised by link adaptation, a base station requires the feedback of certain information from a mobile station with which the base station is communicating. It is desirable, however, to limit the amount of feedback bits assigned to a feedback channel. As such there is a trade-off between the amount of feedback and the performance improvements that can be achieved.
In one feedback technique, the mobile station selects the transmission MCS and feeds this selection back to the base station. There are, however, several disadvantages with this approach. For example, the base station may have a set of operational conditions (including packet size, target packer error rate, etc.) that are different from the assumptions made by the mobile station when the mobile station made its selections. Thus, the base station has no way of knowing if an MCS different from the one selected by the mobile station would be a better choice in view of that operational condition. A second technique is employed in which a mobile station feeds back to the base station three parameters, which the base station uses to calculate the effective signal-to-noise ratio (ESNR) for each transmission mode. This feedback method, however, is inefficient because it requires twenty-four bits of feedback and also because the mobile station must request a feedback channel through medium access control (MAC) requests.